1. Field of the Invention
The present invention is directed to a method of providing a mid-wall repair, as well as a method of evaluating the mid-wall repair.
2. Discussion of the Related Art
In pressurized water (PWR) and boiling water (BWR) nuclear reactors, multiple penetrations are provided in a pressure vessel or piping. The penetrations consist of sleeves and/or nozzles that extend from the exterior of the pressure vessel through openings in a low alloy or carbon steel vessel wall and a nickel-chromium-iron (Ni—Cr—Fe) or stainless steel clad disposed on the interior surface of the pressure vessel. During initial fabrication (i.e., before access to the interior of the pressure vessel is limited, and before the pressure vessel is subjected to radiation and pressurized high temperature water as a result of operation of the nuclear reactor), a J-shaped groove is formed in the vessel interior clad and in some cases the low alloy steel or carbon steel vessel interior wall as well, and a weld material is deposited in the groove to weld the nozzle to the clad and vessel wall, where applicable. Thus, the nozzle is welded from the interior of the pressure vessel to connect the nozzle to the pressure vessel.
As a result of operating and residual stresses in the J-groove weld and the primary water environment during operation, the welds, the sleeves or nozzles, and the Ni—Cr—Fe or stainless steel cladding are subject to stress corrosion cracking. Thus, it becomes necessary to repair the connection between the nozzle and the pressure vessel.
In a known repair technique, the technician does not have access to the highly radioactive interior of the closed pressure vessel. Thus, repair of the connection between the pressure vessel and the nozzle is conducted from the exterior of the pressure vessel.
In the known repair technique, the nozzle is severed at the mid-wall of the pressure vessel and a sacrificial plug installed to create a flush surface at the exterior of the pressure vessel. A welding pad of a material that is not susceptible to stress corrosion cracking, such as Alloy 52, is formed on the exterior of the pressure vessel. A hole is drilled in the welding pad, and a replacement nozzle formed of a material that is not susceptible to stress corrosion cracking, such as Alloy 690, is disposed in the hole. The replacement nozzle is then welded to the welding pad. Because it is not practical to provide postweld heat treatment stress relief of the weld and the adjacent areas, a temper bead welding technique is used to weld the welding pad to the pressure vessel or piping.
The known repair technique suffers from a number of disadvantages, however. These disadvantages include that it is often difficult to precisely align the replacement nozzle with the openings in the pressure vessel wall and the new welding pad on the pressure vessel. Further, a relatively large amount of material is used to provide the welding pad of sufficient size (e.g., 6 inch by 6 inch by 0.5 inch) to permit testing and evaluation of the weld pad to the pressure vessel. Further, because formation of the temper bead must be precisely controlled, the weld pad requires a relatively large amount of time to produce, which may increase down time of the nuclear reactor and the amount of radiation to which the technician is exposed during the repair process. The severity of these problems is compounded by the fact that a typical pressure vessel includes multiple nozzles.